Fixed potential NULL-pointer access when creating a new image

`is_vk_call_successful` returns true even if the result of `vkCreateImage` is `VK_ERROR_VALIDATION_FAILED_EXT`. The specification doesn't mention it as a valid return code, but in some cases it can be returned by the Nvidia drivers anyway, in which case `m_image` will be NULL.

src/wrappers/image.cpp

@@ -620,7 +620,7 @@ bool Anvil::Image::init()
                                                  &m_image);
         }
 
-        if (!is_vk_call_successful(result) )
+        if (!is_vk_call_successful(result) || !m_image)
         {
             anvil_assert_vk_call_succeeded(result);